Syncytium

The field of embryogenesis uses the word syncytium to refer to the coenocytic blastoderm embryos of invertebrates, such as Drosophila melanogaster.

Physiological examples

Protists

In protists, syncytia can be found in some rhizarians (e.g., chlorarachniophytes, plasmodiophorids, haplosporidians) and acellular slime moulds, dictyostelids (amoebozoans), acrasids (Excavata) and Haplozoon.

Plants

Some examples of plant syncytia, which result during plant development, include:

• Developing endosperm
• The non-articulated laticifers
• The plasmodial tapetum, and
• The "nucellar plasmodium" of the family Podostemaceae

Fungi

A syncytium is the normal cell structure for many fungi.

Most fungi of Basidiomycota exist as a dikaryon in which thread-like cells of the mycelium are partially partitioned into segments each containing two differing nuclei, called a heterokaryon.

Animals

Nerve net

The neurons which makes up the subepithelial nerve net in comb jellies (Ctenophora) are fused into a neural syncytium, consisting of a continuous plasma membrane instead of being connected through synapses.

Skeletal muscle

A classic example of a syncytium is the formation of skeletal muscle. Large skeletal muscle fibers form by the fusion of thousands of individual muscle cells. The multinucleated arrangement is important in pathologic states such as myopathy, where focal necrosis (death) of a portion of a skeletal muscle fiber does not result in necrosis of the adjacent sections of that same skeletal muscle fiber, because those adjacent sections have their own nuclear material. Thus, myopathy is usually associated with such "segmental necrosis", with some of the surviving segments being functionally cut off from their nerve supply via loss of continuity with the neuromuscular junction.

Cardiac muscle

The syncytium of cardiac muscle is important because it allows rapid coordinated contraction of muscles along their entire length. Cardiac action potentials propagate along the surface of the muscle fiber from the point of synaptic contact through intercalated discs. Although a syncytium, cardiac muscle differs because the cells are not long and multinucleated. Cardiac tissue is therefore described as a functional syncytium, as opposed to the true syncytium of skeletal muscle.

Smooth muscle

Smooth muscle in the gastrointestinal tract is activated by a composite of three types of cells – smooth muscle cells (SMCs), interstitial cells of Cajal (ICCs), and platelet-derived growth factor receptor alpha (PDGFRα) that are electrically coupled and work together as an SIP functional syncytium.

Osteoclasts

Certain animal immune-derived cells may form aggregate cells, such as the osteoclast cells responsible for bone resorption.

Placenta

Another important vertebrate syncytium is in the placenta of placental mammals. Embryo-derived cells that form the interface with the maternal blood stream fuse together to form a multinucleated maternal-fetal barrier – the syncytiotrophoblast. This is part of creating immune tolerance in pregnancy, that is, to prevent an immune response that would normally result in the rejection of something foreign in the body. The syncytiotrophoblast is made of fetal cells, but prevents expression of antigens on the surface that the mother's immune system would react to, such as HLA. The syncytium serves as an even more selective barrier than would have been possible between cells, such as cells with tight junctions, as for example some blood cells are specialized to be able to insert themselves between adjacent epithelial cells.

Glass sponges

Much of the body of Hexactinellid sponges is composed of syncitial tissue. This allows them to form their large siliceous spicules exclusively inside their cells.

Tegument

The fine structure of the tegument in helminths is essentially the same in both the cestodes and trematodes. A typical tegument is 7–16 μm thick, with distinct layers. It is a syncytium consisting of multinucleated tissues with no distinct cell boundaries. The outer zone of the syncytium, called the "distal cytoplasm," is lined with a plasma membrane. This plasma membrane is in turn associated with a layer of carbohydrate-containing macromolecules known as the glycocalyx, that varies in thickness from one species to another. The distal cytoplasm is connected to the inner layer called the "proximal cytoplasm", which is the "cellular region or cyton or perikarya" through cytoplasmic tubes that are composed of microtubules. The proximal cytoplasm contains nuclei, endoplasmic reticulum, Golgi complex, mitochondria, ribosomes, glycogen deposits, and numerous vesicles. The innermost layer is bounded by a layer of connective tissue known as the "basal lamina". The basal lamina is followed by a thick layer of muscle.